Oxlignin: A Novel Type of Technical Lignin from Kraft Pulp Mills

Abstract

Lignin, a bio-originated polymer, is being explored as an alternative to nonrenewable fossil resources. It is obtained from biomass during pulping and is mostly burned for energy. In most kraft pulp lines, residual lignin in the pulp is oxidized and solubilized during an oxygen delignification step. This study proposes an isolation method for lignin solubilized during oxygen delignification, which we refer to as "oxlignin", and explores its structural characteristics and properties. The study found acid precipitation to be an effective method for partially isolating oxlignin from the oxygen delignification step. Various analytical methods were employed, including UV-vis absorption analysis, ³¹P NMR spectroscopy, FT-IR spectroscopy, SEC, and TGA. In addition, the solubility of the lignin was studied in four different solvents and compared to the commercial kraft lignins. The study found that oxlignin is a promising substitute for lignosulfonates in certain applications due to its hydrophilicity and high solubility in water, methanol, and ethanol. Compared to kraft lignins, oxlignin has a lower phenolic group content but higher carboxylic acid content.

Figure 1

Example of structural changes in lignin during oxygen delignification.

Figure 2

Schematic representation of the various processes involved in a kraft mill, along with the products that can be extracted.

Figure 3

Process of separating and obtaining lignin from the oxygen delignification filtrate.

Figure 4

Solubility in g/L of oxlignin, LignoBoost lignin, and Indulin in water, methanol, ethanol, and ethyl acetate. Samples were analyzed in duplicates.

Figure 5

(a)TGA curves in the O₂ environment and (b) N₂ environment for oxlignin, LignoBoost lignin, and Indulin AT lignin. The corresponding DTG graphs are shown in the figures.

Figure 6

Concentrations of functional groups in mmol/g lignin in oxlignin, LignoBoost, and Indulin based on ³¹P NMR spectroscopy.

Figure 7

FT-IR spectra of oxlignin, LignoBoost lignin, and Indulin in the range of 600–2000 cm^–1.

Figure 8

(a) UV–vis absorption spectra of the soluble lignin after acidification and the starting material, oxyliquor, as well as (b) three dissolved lignin samples (oxlignin, LignoBoost lignin, and Indulin AT).

Figure 9

Size exclusion chromatography of the oxlignin, LignoBoost lignin, and Indulin.

Figure 10

Phase stability of bentonite suspensions with oxlignin and lignosulfonate. Bentonite was dispersed in water (A) or with either oxlignin (B) or a commercial lignosulfonate (C), and phase separation was observed. Photograph courtesy of Carl Moser. Copyright 2025.